Expansion tank? Overpressure? Water heater explosion?

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e9999

e9999

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well, that was interesting.
Still in the middle of replacing my water heater. Long story...
Anyway, I'm taking various pipes apart etc and then it dawned on me. There is no visible expansion tank in that system that I can see. So either it's buried in a wall or hidden in the attic or crawlspace someplace, or the plumber (I guess, if that) did not put an expansion tank in at all. So I'm amazed that the system did not fail long ago from overpressure. There is of course a pressure relief valve on the heater -set at 150psi IIRC- but I never saw that one release water.
Of course, that might explain why I had to change several toilet bowl valves over the years.
Then in addition to that: the main house water regulator failed. That will be a bear to change.


Moral of the story: check that you have a tank in, put a pressure gauge easily visible someplace and keep an eye on that once in a while...
 
The only house I've seen one in had an enclosed loop hot water recirculation system with check valves, and I've worked on others that had the same and didn't.:meh:

But they do work well for water hammer..
 
You don't need an expansion tank. You are thinking of closed-loop systems for baseboard heating.

If you are on a well (I don't think you are) then you will have a pressure tank to store pressure in the house system and keep the well pump from running often. But if you're on "city water" then you don't need this either.

An expansion tank doesn't change the pressure in the system at all, so it won't keep stuff from blowing up.
 
i didnt have one....and had never seen one on the domestic water..just the heating system. power company came to check my boiler. while he was at it, he looked at the (seperate) hot water heater and said it would be good if i had one. so i put one in. havent noticed any change.
 
ff347 said:
I coud type it all out, read it here and a million other places on the web.

What this link says!


Why You Need a Water Heater Expansion Tank from an Atlanta Plumbing Pro | Scott Plumbing
Click to expand...

I read the link but I'm not convinced (unless the code requires it as he says). All hot water heaters are required (by ASME code) to have a pressure relief valve and the discharge side of the relief valve has to be plumbed to a safe location. I don't know why you would need any more protection than that.
 
OK, I did some more research and consulted a plumber out at the Habitat house today. I've never seen one on the domestic hot water system, because all the places I've worked on are either older (no BF preventer) or are on a well, so they wouldn't do anything. The duplex I'm building now is on well, so the well pressure tank accomplishes the same thing.

BUT. Turns out if it's a new house on municipal water, with any type of check valve or back-flow preventer installed at the service (generally now required everywhere), it's a good idea. It's not a good idea to rely on the relief valve to keep things safe, as they are set for pretty high pressure points, and are not intended to be activated very often.

Plumber told me about a house he went to a few years ago that was on city water, they couldn't get a water heater to last more than a couple of year, the tank would start leaking. He installed an expansion tank and no failures since.

Carry on.
 
KLF said:
snip
An expansion tank doesn't change the pressure in the system at all, so it won't keep stuff from blowing up.
Click to expand...


Well, to finish that part off, I don't think that is correct. The expansion tank will indeed keep the pressure lower if it can accommodate thermal expansion of the water.
Consider a rigid pipe system that is completely full of water and you heat the water. The pressure will rise to very high levels, perhaps to the point of pipe failure. Now, have some air in the system or some expandable pipes, if the air pocket is larger than the additional water volume due to expansion, the pressure will not rise much at all.

I suspect the reason why many people don't have one and don't see failure is simply because-other than well or no BF preventers- many systems likely have air pockets in them that just stay there and sort of do the job. Also most folks don't have a pressure gauge in there so the pressure may be very high without them knowing it.
 
WarDamnEagle said:
I read the link but I'm not convinced (unless the code requires it as he says). All hot water heaters are required (by ASME code) to have a pressure relief valve and the discharge side of the relief valve has to be plumbed to a safe location. I don't know why you would need any more protection than that.
Click to expand...


Think about this. The pop off valve is designed to do its job at 150psi, your house plumbing is "generally" designed to be under 80psi. Sure the pressure from expansion of the water heating is not that high all the time, but would you put 70psi more air in your tires occasionally and not expect something to eventually go wrong? Expansion tanks just "absorb" that extra after the heating cycle and give it somewhere to go, until someone in your house opens a water source and the pressure is relieved.
 
Typically you won't find one unless there is a backflow preventer on the supply. WIthout the backflow preventer the water expands back into the supply line running out to the street.

Where you get into trouble is with a backflow preventer - which is required by code in more places over the past few years. Then the domestic water system becomes a closed system and if the water heater malfunctions you can build up pressure. The expansion tank gives this extra pressure someplace to go without blowing the relief valve.
 
ff347 said:
Think about this. The pop off valve is designed to do its job at 150psi, your house plumbing is "generally" designed to be under 80psi. Sure the pressure from expansion of the water heating is not that high all the time, but would you put 70psi more air in your tires occasionally and not expect something to eventually go wrong? Expansion tanks just "absorb" that extra after the heating cycle and give it somewhere to go, until someone in your house opens a water source and the pressure is relieved.
Click to expand...

I understand the theory; my undergrad is in chemical engineering. I just don't think there's enough empirical evidence to justify installing one. Systems typically aren't that tight (even with a check valve), you aren't heating that much water up at one time without opening a valve and two drops of water in a closed system will relieve a lot of psi.

We owned a house in the late 80s in Augusta Georgia and the supply pressure was 125 psig. FWIW, 1/2" Type L copper (annealed) is good to 590 psi and drawn is good to 1105. Now if your house has pvc or cpvc piping then maybe you should install two expansion tanks. :-)

Frankly, if you want one then go for it. I'm guessing 99% of the houses in the US don't have one and typically have no problem...but that's only a guess.
 
PAToyota said:
Typically you won't find one unless there is a backflow preventer on the supply. WIthout the backflow preventer the water expands back into the supply line running out to the street.

Where you get into trouble is with a backflow preventer - which is required by code in more places over the past few years. Then the domestic water system becomes a closed system and if the water heater malfunctions you can build up pressure. The expansion tank gives this extra pressure someplace to go without blowing the relief valve.
Click to expand...

What he said, New water meters have backflow preventers and you need a expansion tank with one. Most municipal water companys are changing over all their meters to the backflow preventer ones but it's up to the home owners to put in an expansion tank. If your house is on a well then you don't need one.
 
WarDamnEagle said:
I understand the theory; my undergrad is in chemical engineering. I just don't think there's enough empirical evidence to justify installing one. Systems typically aren't that tight (even with a check valve), you aren't heating that much water up at one time without opening a valve and two drops of water in a closed system will relieve a lot of psi.

We owned a house in the late 80s in Augusta Georgia and the supply pressure was 125 psig. FWIW, 1/2" Type L copper (annealed) is good to 590 psi and drawn is good to 1105. Now if your house has pvc or cpvc piping then maybe you should install two expansion tanks. :-)

Frankly, if you want one then go for it. I'm guessing 99% of the houses in the US don't have one and typically have no problem...but that's only a guess.
Click to expand...

I would say that about 10% of the houses I've seen that the meter was changed out to the backflow preventer type start poping the relief valves. I think it has to do with the size of the water heater, and how tight the system is (faucet drippin or toilet running on)
 
my pressure reducing valve failed unbeknownst to me. Pressure in the whole house system was 140 psi cold . No expansion tank. No backflow preventer that I know of. Water heater failed not too long after that, although admittedly being very old and corroded.
One point to consider of course is that the pressure relief valve on the tank may also not work properly. And that a failure of a pipe in a wall someplace or in the attic would not be good.
I think I'm putting a tank in just in case.

I do disagree by the way with the notion that one does not heat up much water at a time. Typical shower is probably like 15 gallons (30 for a gal? :) ).
That's replaced by cold water which is then heated.
density of water at 20C: 998kg/m3
at 60C: 983
diff is 1.5%.
So 20 gals become 20.3 gallons (roughly, it's not that simple). A closed system would have to absorb an extra 0.3 gallons of water volume. That's quite a bit.

My gut feeling is that a simple pressure reducing valve would effectively work as a backflow preventer, though. I'll have to look into that.
 
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about 40 states in the us have adopted the International codes (IRS, IBC,IPC...ETC)for building standards.the codes started to be implimented about 2000. expansion tanks for hot water heaters are required in residences for states adopting in international codes.the expansion tank allows the hot water a place to go when the water expands due to heating, usually when the hot water is being stored in the tank...like at night. its supposed to stop water backing up into the potable water system through the cold water supply.

a pressure reducing valve (prv) is usually put on the potable water line right after the main shutoff to the house on the potable water line supplied by the municipality.the prv should regulate water pressure to the house, while the expansion tamk keeps hot water from backing up into the cold water line feeding the house. usually there is no backflow prevention device put onto the system unless there is a specific set of circumstances that require it.

the expansion tank is a fairly recent code change and unless you live in a newer home you wouldn't see it on the hot water heater.
 
Done.
I put in a 2 gallons expansion tank for peace of mind.
 
porten1134 said:
about 40 states in the us have adopted the International codes (IRS, IBC,IPC...ETC)for building standards.the codes started to be implimented about 2000. expansion tanks for hot water heaters are required in residences for states adopting in international codes.the expansion tank allows the hot water a place to go when the water expands due to heating, usually when the hot water is being stored in the tank...like at night. its supposed to stop water backing up into the potable water system through the cold water supply.

a pressure reducing valve (prv) is usually put on the potable water line right after the main shutoff to the house on the potable water line supplied by the municipality.the prv should regulate water pressure to the house, while the expansion tamk keeps hot water from backing up into the cold water line feeding the house. usually there is no backflow prevention device put onto the system unless there is a specific set of circumstances that require it.
the expansion tank is a fairly recent code change and unless you live in a newer home you wouldn't see it on the hot water heater.
Click to expand...



All the municipal water companies in this area require backflow preventers.

Here's a partial list of cities that require them
Backflow Prevention Links
 
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